Part B challenge cohorts will include at least 14 and up to 24 volunteers per vaccine treatment group, recruited among those participating in Part A; at least 4 and up to 12 placebo recipients recruited among those participating in Part A; plus additional unvaccinated control volunteers sufficient to enroll up to a total of 60 recruited, as needed. Volunteers in the Phase 2b study will be enrolled and challenged in 2-4 cohorts due to the CIR Inpatient Unit capacity of 30 beds. A minimum of 8-10 controls will be challenged with each cohort of vaccinees to ensure comparability of attack rates across challenge cohorts. Volunteers will be admitted as inpatients and challenged, with approximately 2 x 10^7 cfu of the fully virulent ETEC strain, H10407, followed by 5 days of direct observation. A Data Review Committee (DRC) will be convened to review the clinical data for all challenged volunteers and verify all outcomes per protocol definitions before any vaccine efficacy assessments are made.